I did all the work done myself - it took a while (about a year), with the slow progress mainly down to my complete lack of relevant experience.
I got there in the end though, which just goes to show that this is the kind of project that can be handled by a ham fisted DIYer like myself, you will need a lot of patience, access to a bit of help for the heavy lifting, and a willingness to give it a go.
The designs were done in Sketchup, and the building is constructed with the materials typically used in commercially available garden rooms: structurally insulated panels, cedar cladding, 'mini plinths' footings and epdm rubber on the roof.